From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Aircraft valve mechanisms, material requirements, and operational issues like seating velocity.
| Identifier | ExFiles\Box 158\5\ scan0022 | |
| Date | 1st March 1939 | |
| March, 1939 AIRCRAFT-VALVE MECHANISMS 113 compression chamber past the check ball. When the cam starts to lift, the plunger builds up pressure in the compression chamber, forcing the ball to seat, and the valve is then lifted on a column of oil. During the lift period a predetermined slight leakage occurs between the plunger and cylinder bore. If no change has occurred in the distance between the valve seat and cam, this small amount is replaced before the valve lifts again. However, if, for example, the valve has expanded then the refill occurs only to the extent to cause zero clearance, and the plunger is at a lower position compensating for the decrease in the overall length. The forces existing to hold the valve off its seat are the plunger spring and the oil pressure acting on the plunger, the sum of both being much less than the valve-closed spring load. With this maintenance of no clearance, it is obvious that the valve motion – provided there is no false motion – is always going to follow the cam contour and assure the designer that the seating velocities are fixed as originally intended. In addition to this factor, the timing must remain constant since the tappet or roller, as the case might be, will lift the valve at the exact point and not have to take up clearance before lifting or closing the valves. With fixed timing point, more latitude can be taken with the cam design, resulting in obtaining a better lift profile, that is, more lift at a given crank angle with consequent better filling or scavenging without resorting to high acceleration with its attendant evils of high loads, causing deflections, and so on. One extremely important factor that we can only discuss at this time from the theoretical but extremely practical viewpoint is the elimination of wide overlaps existing between cold starting conditions and full-throttle operation in the radial air-cooled engine with pushrod-operated valve mechanism. Fig. 11 represents the relation between inlet-valve opening and exhaust closing of the type engine that has the clearance change illustrated as "aircraft" in Fig. 9 at idling or cold conditions. When the inlet has opened an appreciable amount, it will be noted that the exhaust valve is still at a point of considerable lift which obviously will affect the operation of the engine. This means that the designer laying out the timing must necessarily compromise as he is never sure just where the valve is going to start to open or close; consequently some sacrifice is made on the cam pro-file to accommodate the wide angles necessary to cover an extremely wide range of clearance allowance. Valves Valve-material requirements have been studied since the introduction of the poppet-valve type of engine, and all of the authors in their conclusions are unanimous regarding the characteristics required as far as temperature effects are concerned. Dr. Leslie Aitchison, in a paper presented to the Institution of Automobile Engineers² in 1920, stated a number of requirements that a valve steel should possess, several of which are as follows: 1. The greatest possible strength at high temperatures. 2. The highest possible notched-bar value at high temperatures. 3. The least possible tendency to scale and, if scaling does occur, the scale should be as adherent as possible. 4. The ability to retain its original physical properties after frequent heatings to high temperatures followed by cooling to normal temperatures, also after being heated to an elevated temperature for a considerable length of time. 5. No liability to harden when cooled in air from the temperature which it will attain when used normally as a valve on an engine. In 1931, A.{Mr Adams} L. Boegehold and J.{Mr Johnson W.M.} B. Johnson³ suggested, as outstanding requirements: ² See Proceedings of the Institution of Automobile Engineers, Vol. 14, 1920, p. 39: "Valve Failures and Valve Steels in Internal-Combustion Engines," by Leslie Aitchison. ³ See "Engineering Requirements in the Automotive Industry for Metals Operating at High Temperatures," by A.{Mr Adams} L. Boegehold and J.{Mr Johnson W.M.} B. Johnson, joint ASME and ASTM symposium on metals at high temperature, June 23, 1931. HIGH SEATING VELOCITY AND OUT-OF-SQUARE CYLINDER SEAT HIGH SEATING VELOCITY WITH SPIN Fig. 8 – The result of out-of-squareness and high velocity is illustrated by the appearance of the upper and lower valve seats | ||